The present invention relates to anti-lock (ABS) braking systems for road vehicles and is concerned in particular with the so-called select-low logic of ABS rear axle control.
It will be helpful to the understanding of the following discussion of the present invention to identify and distinguish between the two basic types of control systems conventionally used to control the supply of pressurised fluid to the brake actuator(s) of the braking system. The first type is normally referred to a solenoid/solenoid or Two Solenoid per channel and the other type is normally referred to as the Flow-Valve type. The basic construction and function of these two systems is now described.
In a Solenoid/Solenoid or Two Solenoid per Channel system two solenoids control two fluidic valves adapted to control firstly the communication between the pressure source (master cylinder) and a brake actuator, and secondly, the communication between that brake actuator and a low pressure reservoir (tank or expander) which usually forms the inlet to a return pump adapted to supply fluid under pressure to the pressure source. The first valve is commonly known as the inlet valve, the second valve thus being the outlet or as more commonly known, dump valve. The inlet valve has two main functions, (1) to block communication between the pressure source and the brake actuator when the brake pressure is being reduced by the dump valve, and (2) to control the rate at which the brake pressure is reapplied to the brake actuator during the apply phase by control of the solenoid pulse rate. An early example of this type of control system can be found in GB-A-1 243 523.
On the other hand, in a Flow-Valve system the inlet valve of the above described Solenoid/Solenoid system is replaced by a spool valve controlled by the presence of a pressure differential across it. The control functions it performs are substantially identical to that of the solenoid controlled inlet valve above, i.e. blocking of the inlet connection and application rate control, except that the flow valve is arranged to provide substantially a constant flow rate, i.e. fixed reapplication rate. Variation in this rate can be achieved by pulsing of the dump valve, but for the majority of operational conditions the tuned fixed rate is satisfactory. An example of this type of control system can be found in our EP-A-202845.
One major difference with respect to the reapplication flow rate is that the flow valve rate is substantially independent of the applied pressure of the source, whereas in the 2 solenoid per channel system for a given inlet solenoid pulse rate, the actual reapplication flow rate will vary in accordance with the applied pressure of the source. Usually the ABS system has no knowledge of the applied pressure of the source and therefore the only system that can reliably produce a desired reapplication rate is the flow valve system.
Most passenger car ABS are arranged to provide independent control of the brake pressure applied to each front wheel, but Select-Low control of the rear axle. Select-Low control entails controlling the brake pressure to both rear wheels with identical solenoid firings based on the behaviour of whichever one of the pair has the greatest tendency to skid. The objective is to maintain pressure equilibrium across the axle. Independent control of all four wheels would provide optimum deceleration, but inadequate stability during manoeuvres, and on split .mu. surface, that is where one wheel of a given axle is running on a high .mu. surface such as a dry road and the other wheel of that axle is running on a relatively low .mu. surface such as ice.
Trucks often have individual control of each wheel, or group of wheels, but are inherently more stable because of their longer wheelbase. Some trucks have Select-Low control at the front axle in order to reduce steering kickback caused by the use of steering geometry with a large ground-offset; it is also known to change gradually from Select-Low to independent control so that stopping distance can be reduced.
In the context of a 2-Solenoid per Channel system, a Quasi-Select-Low logic has already been proposed for the rear axle, whereby the Select-Low pressure dump firings for a still-stable rear wheel are delayed until either the opposite wheel regains stability, or a fixed maximum period has elapsed. Immediate solenoid firing is permitted if the wheel becomes unstable during the delay period.
The principal objective in the latter system was to improve the pedal feedback by staggering the scavenging of the dumped brake-displacement volume. During the delay period, the brake pressure for that wheel is held constant, provided that the wheel remains stable. At the end of the delay period the dump solenoid of the stable rear wheel is fired for a period corresponding to the aggregate dump time of the unstable rear wheel, so as to re-establish pressure equilibrium across the axle. The brake pressure at both wheels is then increased synchronously until the next skid cycle occurs.
A secondary objective of the known Quasi-Select-Low system is to reduce stopping distance on non-homogeneous surfaces, but this aim is compromised to some extent by the need to achieve pressure equilibrium before commencing the reapply process.
Using the brake pressure hold facility of the latter known system, it would have been attractive to maintain constant pressure at the still-stable wheel throughout the modulation of pressure at the unstable wheel, and then to calculate the number of reapply pulses needed after the pressure dump in order to reestablish the original pressure level. Unfortunately, this is not practicable in a Two-Solenoid per Channel system because the magnitude of each reapply pulse depends upon the pressure level in the pressure source or master-cylinder, which can vary unpredictably during the course of the stop.
Thus, in a Two-Solenoid per Channel system, calculation of the number of re-apply pulses needed at the controlled, low-mu wheel to bring its pressure approximately back to that of the co-controlled, higher-mu wheel is extremely difficult and in practice cannot be easily achieved, due to the number of variables controlling fluid flow within the system.
The known Quasi-Select-Low logic works in that the dump time needed to bring the co-controlled wheel's pressure down to that of the controlled wheel is relatively straight forward to calculate because they both started from the same brake pressure; the subsequent re-apply phases can then be kept in synchronism by the use of identical hold-value pulse widths because they are both supplied by the same master-cylinder pressure, albeit from separate circuits. However, potentially-useful brake force at the still-stable wheel has to be surrendered whilst its brake pressure is first dumped to a relatively low level, then increased at a cautiously slow rate.
When considering how to achieve the same objectives within the context of a Flow-Valve system, the prior art predicts only problems, not solutions. The hold facility is crucial to the prior-art logic, both whilst sustaining the pressure at the still-stable wheel during the delay period, and whilst maintaining the final dump pressure in the previously-unstable wheel until the opposite wheel has been dumped to that new equilibrium level.
It is possible to maintain the pressure level of a flow-valve system within quite a narrow range by pulsing the dump solenoid, but only when the appropriate mark-to-space ratio is known, or can be found by observation. Because the required mark-to-space ratio will vary considerably with brake pressure, which the system could only estimate with considerable approximation, the accuracy of the quasihold is inadequate for this purpose.
If the pressure at the still-stable wheel cannot be held constant, then it will continue to rise during the delay period, and this raises concerns about potential impairment of vehicle stability and how to calculate the extra dump-time needed to compensate for the unwelcome additional pressure. These are the questions raised by analysis of the prior art for which no solution has until now be proposed.